1. Field of the Invention
The invention relates to the field of microalgae and cyanobacteria, specifically how microalgae and cyanobacteria may be cultivated to produce biofuels, food for humans and animals, organic fertilizers or pharmaceutical products.
2. Description of the Prior Art
Due to the increasing costs of fossil fuels as well as its negative impact for the environment (e.g. climate change) there is a growing interest for the use of biofuels such as biodiesel and bioethanol. So far, these biofuels have been obtained from traditional oleaginous and cellulose rich feedstock. However, these traditional crops are not cost effective and the biofuel production yield is not high enough to compete with petroleum and its derivates. Therefore, the production of biofuels from traditional crops may lead to negative side effects such as intolerable rises in food prices and other global issues.
One of the most promising solutions to solve the fossil fuel scarcity and its associated environmental issues is the production of biofuels from microalgae which are single cell photoautotrophic microorganisms. Photoautotrophic organisms (usually plants) carry out photosynthesis to acquire energy from sunlight to convert carbon dioxide and water into organic materials to be used in cellular functions such as biosynthesis and respiration. Importantly, single cell microalgae living in an aqueous solution transform light into chemical energy much more efficiently than any other organism due to their greater access to carbon dioxide and dissolved minerals. In addition, single cell microalgae are able to store lipids in higher density than any other plant or multi-celled organism which requires time, energy and nutrients to build support structures such as roots and stalks, light collector structures such as leaves, and lipid storing organs such as seeds. Therefore, to produce biofuels from microalgae presents the following advantages with respect conventional crops, if cultivated at a large scale:                Production yields are 30 to 100 times higher for micro-organisms than any other known traditional crop.        Microalgae can grow in soils and tolerate water that is not useful for conventional agriculture. They can even tolerate the use of waste water and saltwater.        During its growth, enormous amounts of carbon dioxide as well as other contaminant gases are captured. In addition, vast amounts of oxygen are produced and liberated to the atmosphere.        
Microalgae are single cell organisms that convert sunlight into chemical energy through photosynthesis:

Microalgae and cyanobacteria are the planet's most abundant organisms, having adapted to extreme conditions such as polar and volcanic environments. They constitute the core of the trophic chain that sustains life on Earth as well as of the natural carbon cycle. They produce 80% of the planet's biomass (phytoplankton).
Some algae and cyanobacteria mass concentrations of lipids that may achieve proportions in the range of 60-70% by weight on a dry basis. Therefore, they are ideal to produce biodiesel. In addition, algae constitute an effective and powerful carbon sink. It has been demonstrated that 100 tons of algae biomass will capture 170 tons of carbon dioxide.
In addition to light, carbon dioxide and water, photosynthesis requires inorganic salts which include essential elements such as nitrogen, phosphorous, iron and in some cases, silicon. The optimal temperature for microalgae growth is between 20 and 30° C. Therefore, to cultivate microalgae and cyanobacteria, the following is required:                An aqueous media containing the algal culture, which can be fresh water, brackish water or saltwater depending on the organism type.        A light source in order to develop the photosynthetic process.        A carbon dioxide source to enhance the photosynthetic process.        A system to extract the oxygen produced during the photosynthesis.        Nutrients (mineral salts).        A system to move or circulate the aqueous media containing the algal culture to enhance the photosynthesis.        
Today, there are two main types of photobioreactors to grow microalgae on a large scale:
Raceways or open ponds. These systems are very simple. They are composed of a circulating pond or a set of circulating channels open to the atmosphere in which the aqueous solution circulates while capturing the sunlight. The biggest advantage of these photobioreactors is that they are very economical. However, open photobioreactors do not easily sustain the conditions for desired microalgae and cyanobacteria growth because the conditions of the algal culture can vary substantially over time due to water evaporation. In addition production is affected by contamination with unwanted algae and microorganisms that are deposited on the algal culture.
Closed photobioreactors have been developed in many different typologies to overcome the issues found in open ponds. Closed photobioreactors are based on closed hydraulic circuits, mostly tubular, through which the algal culture circulates. This type of system allows more intensive algae growth, requires less land surface and does not present contamination risks. However, with this technology, the oxygen produced during the photosynthesis, which is toxic to the algae and bacteria, may become an issue and might be hard to eliminate. Additionally, the cost of installation is around ten times higher than the cost of raceway photobioreactors. Due to the high costs, closed photobioreactors are not yet economically viable.
Current photobioreactors present important limitations to producing microalgae and cyanobacteria at a large scale. Whereas some of them require enormous amounts of energy to operate, others present prohibitive construction, installation and maintenance costs.
Today, the bottleneck for the production of massive amounts of microalgae and cyanobacteria is the photobioreactor itself. What is needed is a developed photobioreactor that makes it technically and economically feasible to produce microalgae and cyanobacteria on a large scale and ultimately makes producing biofuels competitive with that of fossil fuels.